Paper EH-WeP4
Imaging of dry-out in a LiCoO₂ Cathode in a Li-ion Battery

Wednesday, December 14, 2016, 4:00 pm, Room Mauka

Lithium ion batteries have attracted an attention in the field of portable electronic devices and electric vehicles due to high energy density, high output voltages, and no memory effect. Therefore, Li-ion batteries are recognized as promising candidates for energy storage.

The main elements of Li-ion batteries are positive electrodes, negative electrodes, separators, and electrolytes. LiCoO₂ and graphite that are layered materials are commonly used as electrode materials for Li-ion batteries.

Lithium ion batteries operate through migration of Li-ions between the electrodes. During charging and discharging, Li-ions migrate between LiCoO₂ and graphite, and Li-ions are inserted into or extracted from particles of electrode materials. Insertion/extraction of Li-ions induce changes in volume. The volume change causes dry-out of electrolyte.

Dry-out of electrolyte is one of the most important issues in Li-ion batteries. The volume changes of the electrode materials induce dry-out of electrolyte, which greatly degrades the batteries. Our samples to investigate dry-out were as follows. LiCoO₂ particles around 10 µm in diameter were coated with the Al current collector. The thickness of the coated layer was around 30 µm. A 20-µm-thick polypropylene separator with the electrolyte was sandwiched between the LiCoO₂ cathode and the graphite anode.

We observed volume changes in the LiCoO₂ cathode using SPM. The observed volume changes are caused by electrolyte flux induced by changes in concentration of Li-ions. However, the volume changes were greatly reduced when the electrolyte dried out. The dry-out and infiltration of electrolyte between the LiCoO₂ particles and the current collector spread out with the procedure of degradation of the batteries. The boundaries between the dry-out and infiltration regions acted as barriers of electrolyte flux.